This invention relates to apparatus for maintaining excitation during excessive output current conditions, i.e. during overload and/or fault conditions, of a generator whose field winding is normally supplied with power from a shunt-type regulator.
The voltage and current for the field winding of an alternating current generator are often supplied from a shunt-type regulator, i.e. from a regulator that receives its power from the generator itself. This power from the regulator is supplied either directly to the generator's field winding or to the field winding of an exciter. In the latter case the output of the exciter is in turn supplied to the generator's field winding.
Generator systems which use shunt regulators are quite satisfactory during normal operating conditions, but they do have some problems during fault or overload conditions. When a fault or overload occurs, the current in one or more of the generator's output lines increases and the voltage, which is also supplied to the regulator, falls. As a result the output of the regulator decreases, causing a decrease in the strength of the field winding's field which further reduces the output voltage of the generator. This feedback can quickly lead to a complete collapse of the generator's field and an accompanying loss of generator output. In some instances, the collapse occurs before breakers or other protective devices near the fault can clear it.
Compounding this problem of generator systems that use shunt-type regulators is the fact that different fault or overload conditions require different field winding voltages to maintain generator excitation. For example, in the case of a three-phase generator, three different voltages are required to maintain excitation during a line-to-neutral short, a line-to-line short, and a three-phase short, respectively. Although impressing the largest of these voltages upon the field winding during any of the faults will maintain excitation of the generator during all three fault conditions, it also results in excessive short circuit current flowing in the output lines in all fault conditions except the three-phase short, until the breakers clear the lines.
Prior art generator systems with shunt-type regulators are shown in U.S. Pat. Nos. 2,454,582 and 3,316,499.